How to force postfix to use secure smtp?

Dear all ISPConfig3 Postfix users,
Since I need to send out account passwords to users by emails, hacker may intercept these credentials when listening to the tcp out port 25 of my ispconfg server.

Now my question is: how to force postfix to use secure smtp to send out email? That is to secure the path from my ispconfig3 server to my customer's email server. For instance, if he is a msn user, the picture becomes: